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1

Local Ionospheric Modeling Using the Localized Global Ionospheric Map and Terrestrial GPS

Sharifi M. A., Farzaneh S.

Acta Geophysica

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2016

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Vol. 64, no. 1

237--252

EN

Global ionosphere maps are generated on a daily basis at CODE using data from about 200 GPS/GLONASS sites of the IGS and other institutions. The vertical total electron content is modeled in a solargeomagnetic reference frame using a spherical harmonics expansion up to degree and order 15. The spherical Slepian basis is a set of bandlimited functions which have the majority of their energy concentrated by optimization inside an arbitrarily defined region, yet remain orthogonal within the spatial region of interest. Hence, they are suitable for decomposing the spherical harmonic models into the portions that have significant strength only in the selected areas. In this study, the converted spherical harmonics to the Slepian bases were updated by the terrestrial GPS observations by use of the least-squares estimation with weighted parameters for local ionospheric modeling. Validations show that the approach adopted in this study is highly capable of yielding reliable results.

2

Symmetric neutral-atmosphere mapping functions: a review of the state-of-the-art

Sharifi M. A., Souri A. H.

Artificial Satellites : Journal of Planetary Geodesy

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2013

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Vol. 48, No. 4

159--170

EN

The aim of this paper is to review of six recent symmetric mapping functions. The mapping function can be largely used for GPS meteorological measurements, InSAR atmospheric corrections and precise measurements of very long baseline interferometry (VLBI). These spacebased techniques use radio signal that propagate through the Earth's atmosphere. The electrically-neutral region, predominantly the troposphere, affects the speed and direction of travel of radio waves leading to existence of excess path. The mapping function models the elevation angle dependence of the delay. Within the past decade, significant improvements have been achieved in order to use of Numerical Weather Models (NWM) for geodetic positioning. Ray-tracing algorithms have been performed through refractivity shells retrieved from NWMs in order to relate zenith delays to slant delays. Therefore, there seems to be a real need for deep review of recent developments in the mapping function domain. This paper proposes a comprehensive review of the symmetric mapping functions state of the art, their spatio-temporal variations and used NWM and generic models. Niell Mapping Function (NMF), Vienna Mapping Function (VMF1), University of New Brunswick-VMF1 (UNB-VMF1) mapping functions, Global Mapping Function (GMF) and Global Pressure and Temperature (GPT2)/GMF are reviewed in this paper.

3

Rosborough formulation in satellite gravity gradiometry

Sharifi M. A., Safari A., Far K. G.

Artificial Satellites : Journal of Planetary Geodesy

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2013

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Vol. 48, No. 1

39--50

EN

Following the launch of CHAMP, a new era was born in the gravity field determination from satellite observations. Many methods have been proposed and applied for the recovery of the Earth’s gravity field from the observations of the satellite missions CHAMP, GRACE and GOCE. This paper deals with the Rosborough formulation in gravity field modelling. This formulation is derived from the transformation of time-wise representation from the orbital into the spherical coordinate systems. Base functions of the Rosborough formulation depend on the type of the functional of the gravity field and the inclination of the orbit. Unlike the space-wise approach, the Rosborough approach can easily deal with both isotropic and non-isotropic functionals. The proposed formulation is implemented on the GOCE data in order to show its efficiency. Numerical results show that the Rosborough formulation is a powerful and efficient tool in the case of GOCE gradiometry data processing.

4

Evaluation of symmetric neutral-atmosphere mapping functions using ray-tracing through radiosonde observations

Souri A. H., Sharifi M. A.

Artificial Satellites : Journal of Planetary Geodesy

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2013

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Vol. 48, No. 4

171--189

EN

The aim of this paper is to compare the validity of six recent symmetric mapping functions. The mapping function models the elevation angle dependence of the tropospheric delay. Niell Mapping Function (NMF), Vienna Mapping Function (VMF1), University of New BrunswickVMF1 (UNB-VMF1) mapping functions, Global Mapping Function (GMF) and Global Pressure and Temperature (GPT2)/GMF are evaluated by using ray tracing through 25 radiosonde stations covering different climatic regions in one year. The ray-traced measurements are regarded as “ground truth”. The ray-tracing approach is performed for diverse elevation angle starting at 5o to 15o . The results for both hydrostatic and non-hydrostatic components of mapping functions support the efficiency of online-mapping functions. The latitudinal dependence of standard deviation for 5o is also demonstrated. Although all the tested mapping functions can provide satisfactory results when used for elevation angles above 15o , for high precision geodetic measurements, it is highly recommended that the online-mapping functions (UNBs and VMF1) be used. The results suggest that UNB models, like VMF have strengths and weaknesses and do not stand out as being consistently better or worse than the VMF1. The GPT2/GMF provided better accuracy than GMF and NMF. Since all of them do not require site specific data; therefore GPT2/GMF can be useful as regards its ease of use.

5

A comparison between numerical differentiation and Kalman filtering for a LEO satellite velocity determination

Sharifi M. A., Seif M. R., Hadi M. A.

Artificial Satellites : Journal of Planetary Geodesy

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2013

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Vol. 48, No. 3

103--110

EN

The kinematic orbit is a time series of position vectors generally obtained from GPS observations. Velocity vector is required for satellite gravimetry application. It cannot directly be observed and should be numerically determined from position vectors. Numerical differentiation is usually employed for a satellite’s velocity, and acceleration determination. However, noise amplification is the single obstacle to the numerical differentiation. As an alternative, velocity vector is considered as a part of the state vector and is determined using the Kalman filter method. In this study, velocity vector is computed using the numerical differentiation (e.g., 9-point Newton interpolation scheme) and Kalman filtering for the GRACE twin satellites. The numerical results show that Kalman filtering yields more accurate results than numerical differentiation when they are compared with the intersatellite range-rate measurements.

6

Comparison of availability of Galileo, GPS and a combined Galileo/GPS navigation systems

Wang J., Keller W., Sharifi M. A.

Artificial Satellites : Journal of Planetary Geodesy

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2006

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Vol. 41, No. 1

3--15

EN

Galileo satellites have been planned to be launched in 2006 by the European Union. Launching these satellites will double the number of positioning satellites currently accessible to users. As a consequence, accuracy and reliability will be improved dramatically. Besides accuracy and reliability, availability is also used for quantification of a Global Navigation Satellite System performance. This paper briefly describes the constellation of space segment of the proposed Galileo navigation system in comparison with the well-known Global Positioning System (GPS). Availability of GPS, Galileo and Galileo/GPS satellites are then formulated in terms of Satellite Appearance Distribution Function (SADF). Achievable accuracy in horizonal and vertical positions and timing are expressed using the respective Dilution of Precisions based on the derived mathematical formulation.

7

Site selection for waste disposal through spatial multiple criteria decision analysis

Sharifi M. A., Retsios V.

Journal of Telecommunications and Information Technology

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2004

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nr 3

28-38

EN

This article deals with the application of spatial multiple criteria evaluation (SMCE) concepts and methods to support identification and selection of proper sites for waste disposal. The process makes use of a recently developed SMCE module, integrated into ITC's existing geographic information system called ILWIS. This module supports application of SMCE in planning and decision making processes through several compensatory and non- compensatory approaches, allowing inclusion of the spatial and thematic priority of decision makers. To demonstrate the process, a landfill site selection problem around the town of Chinchina, in Colombia, is used as an example. Based on different objectives, a spatial data set consisting of several map layers, e.g., land use, geological, landslide distribution, etc., is made available and used for modeling the site selection process.